CHORD. VIA 



309 



lizards and turtles there is a urinary bladder attached to the 

 ventral wall of the cloaca ; it is absent in the snakes and 



E I)h 



Am 



E Dh 



4h 



y 



3 



i 



(fe,- a 



^^rrf^ 



FlG. 317. Diagrams illustrating the formation of the foetal membranes of a mammal, 

 a, ovum with the embryo just beginning to develop; b, ovum with embryo in longitudinal 

 section and developing amnion; c, ovum with amnion closing and developing 

 d, embryo with mouth and anus; villous serosa ;'e, chorion in contact with yolk 



sac reduced, amniotic cavity enlarged, .-/.ectoderm; Ah, amniotic cavity ; Al, allantois ; 

 l>n, amnion; C/i, chorion (vascular layer of allantois) ; Chz, chorionic villi (consisting of 

 chorion and serosa; D, zona radiata ; D' , villi of zona; Dg, umbilical stalk; Dh, ii 

 tinal cavity; Ds, cavity of embryonic (blastodermic) vesicle, later of the yolk sac (umbili- 

 esicle) ; E, embryo; J, entoderm; M, mesoderm; Sh, serosa; Sz, villi of set 

 r Kolliker, from Claus and Sedgwick's Text-book.) 



